With the recent development of the information society, a content data reproduction system is widely used that delivers to a user terminal content data such as electronic books, newspapers, music, and motion pictures and the like and allows the content data to be available.
Since the electronized content data (hereinafter referred to simply as “content data”) is easily duplicatable, illegal activities disregarding copyright may easily occur. From a point of view of protecting content data from such illegal activities, the content data is usually encrypted with an encryption key and is recorded, while it is decrypted when it is reproduced. Content Protection for Prerecorded Media (CPRM) is one of such content data protection technologies. Another proposed technique is a double-key encryption technique which encrypts a content key doubly by two keys (see e.g., Patent Document 1). A double-key encryption technique of this type is used in, for example, MQbic®. Of the encryption keys, a key unique to a recording medium, for example, a medium-unique key is securely stored in a hidden area of the recording medium and made completely inaccessible from outside. Hence, for example, if a committer of illegal copying illegally copies only encrypted content key data, he or she cannot use the content data unless he or she has the medium-unique key.
However, if the medium-unique key is somehow read out illegally and given away to a host manufacturer who has not got a formal license, it is likely that the content data will be used illegally with an unauthorized device that is manufactured based on the leaked information.